RFR: 8191564: Refactor GC related servicability code into GC specific subclasses

[Roman Kennke]

Hi Mandy,

thanks for reviewing! I think Erik Ö. already pushed it for me.

> A minor comment that GCMemoryManager could take an enum to indicate the 
> type of this  GC action (major vs minor).   This can be a future cleanup.

This may be overly restrictive: some GCs may not necessarily provide a 
major/minor distinction, but have only one GC manager (e.g. epsilon), or 
may provide 3 or more GC managers (e.g. 'minor' partial (or young) GC, 
'intermediate' concurrent GC, 'intermediate or major' concurrent full 
GC, 'major' STW last-ditch full GC...).


Roman